In many cases, it is desired to use a laser beam having a center or nominal wavelength that is determined based on requirements or goals of a particular application environment. For example, it may be desired to generate laser light at the optimal performance band of a detector that is preferred for an application. In other cases, a particular wavelength or wavelength band may be preferred to reduce eye hazards or interference with or from other optical signals. In other cases, it may be desired to analyze absorption or scattering wavelength(s) of interest. Many other examples are possible to illustrate that a wide variety of laser wavelengths are desired for various laser related applications.
Unfortunately, high pulse energy laser light cannot be generated or are not readily available at optimal wavelengths for all applications. Indeed, the highest pulse energy beams can generally only be generated at a number of discrete wavelengths. As a result, wavelength shifters are used for a variety of applications. These shifters receive an input or pump beam having suitable performance characteristics and provide an output beam at a shifted wavelength as desired for the subject application.
One family of wavelength shifters is based on second order non-linear processes and includes optical parametric amplifiers and oscillators (OPAs and OPOs; respectively). These are preferred for many applications because they are in solid state form so they are convenient to use and maintain. However, they generally entail some absorbance and may therefore be damaged, particularly for high power applications (e.g., characterized by high pulse energy).
Because of this, wavelength shifters based on third order non-linear processes including stimulated Raman scattering (SRS) are used for certain applications. For convenience, devices that utilize this third order process are referred to herein as Raman shifters. An illustrative Raman shifter involves illuminating a medium, typically gas in a cell, using a source pump laser. The intense electric field excites molecular vibrations in the medium and the frequency of scattered radiation (the Stokes output) is shifted by the frequency of those vibrations. In at least one case, a Raman amplifier has been seeded at the shifted wavelength using a visible laser diode to improve efficiency. Various output wavelengths can be achieved via SRS shifting by varying the pump laser and medium.
A number of factors limit the performance of such Raman shifters. First, the output intensity is a function of the interaction length or quantity of the medium illuminated. For a given beam diameter and desired medium pressure (in the case of a gas cell), a significant Raman cell path length may be required to provide the desired output intensity (e.g., if the beam intensity for a short path would cause optical breakdown or sooting), thereby adversely affecting instrument compactness. Although the optical path may be folded within the Raman cell, such folding typically results in path overlap that reduces illumination efficiency.
Moreover, path folding is generally accomplished by using coated optics. Such coatings are subject to damage at high pulse energies and generally have performance characteristics that are highly wavelength dependent. That is, such coatings are generally formed from alternating layers of high and low index of refraction materials where the thicknesses of the layers are selected to optimize performance for a particular wavelength, e.g., a quarter-wave stack. As a result, these optics and the Raman cell may be limited to a narrow wavelength band of applications, for example, on the order of 10 nm or less.
Other difficulties of Raman shifters relate to circulating the medium and cell transmission losses. Improved operating performance generally requires that the medium be circulated so that different portions of the medium are illuminated over time, particularly for high power applications. Achieving suitable circulation further complicates design, particularly for folded path geometries. Moreover, the beam is generally transmitted into and out of the cell containing the medium via windows formed in a cell wall. Such transmission through windows may entail optical losses.